Irish singer Sinead O'Connor threatens to sue Miley Cyrus

The 'Nothing Compares 2 U' hitmaker has hit back at the 'Wrecking Ball' singer after she posted a series of messages on Twitter mocking her mental illness and comparing her to troubled starlet Amanda Bynes - who has spent the summer in a psychiatric hospital - and now Sinead has vowed to seek legal action unless the tweets are taken down.

She wrote in an open letter on Friday: "Miley... Really? Who the f**k is advising you? Because taking me on is even more f**kin' stupid than behaving like a prostitute and calling it feminism.

"You have posted today tweets of mine which are two years old, which were posted by me when I was unwell and seeking help so as to make them look like they are recent. In doing so you mock myself and Amanda Bynes for having suffered with mental health issues and for having sought help.

"Remove your tweets immediately or you will hear from my lawyers. I am certain you will be hearing from all manner of mental health advocacy groups also. It is not acceptable to mock any person for having suffered. (sic)".

However, the former 'Hannah Montana' star appeared to be unfazed by the threat and has offered to meet the 46-year-old singer to work through their differences.

She hit back: "Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week.

"So if youd like to meet up and talk lemme know in your next letter. :) (sic)".

The feud between the pair kicked off earlier this week after Sinead posted a letter on her official website warning Miley her provocative behaviour and scantily-clad performances would leave her "prostituted" by the music business.

Miley took to Twitter almost immediately to slam Sinead, writing: "Before Amanda Bynes.... There was...", attaching a screenshot of a tweets from an account she believed was Sinead's, clamouring for psychiatric help.

While the account was a fake, Sinead did post suicidal tweets in 2011 and has struggled with mental illness in the past.